HaXml

Warning! The new stable version (1.20) significantly
changes the API of some modules over the previous stable version (1.13).
(If you have already been using the development versions (1.14-1.19.x),
then you are probably OK.)

HaXml is a collection of utilities for parsing, filtering,
transforming, and generating
XML documents using
Haskell. Its basic facilities
include:

a parser for XML,

a separate error-correcting parser for HTML,

a SAX-like stream parser for XML events,

an XML validator,

pretty-printers for XML and HTML.

For processing XML documents, the following components are also provided:

Combinators is a combinator library for generic XML document
processing, including transformation, editing, and generation.

XmlContent is a replacement class for Haskell's Show/Read
classes: it allows you to read and write ordinary Haskell data as XML
documents (and vice versa). The DrIFT tool (available from
http://repetae.net/~john/computer/haskell/DrIFT/)
can automatically derive this class for you.

DtdToHaskell is a tool for translating any valid XML DTD
into equivalent Haskell types, together with XmlContent instances.

Finally, Xtract is a grep-like tool for XML documents,
loosely based on the XPath and XQL query languages. It can be used
either from the command-line, or within your own code as part of the
library.

An introduction to HaXml for people who know more about XML than
about Haskell can be found at
IBM DeveloperWorks.
Please note that the DeveloperWorks article was based on an older
version of HaXml. If you try to use the examples given there, you
will need a couple of minor but important edits, given as a
diff patch here.

To use -package HaXml interactively with GHCi, you need
at least ghci-5.02.3.

The function toDTD generates Parameter Entity Declarations in the internal
subset of the DTD, which don't conform to the strict well-formedness
conditions of XML. We think the constraint in question is spurious,
and any reasonable XML tool ought to deal adequately with full PEs.
Nevertheless, many standard XML processors reject these auto-generated
DTDs. The solution is easy - just write the DTD into a separate file!

DtdToHaskell generates the Haskell String type for DTD attributes
that are of Tokenized or Notation Types in XML. This may not be
entirely accurate.

To install HaXml, you must have a Haskell compiler: ghc-6.2
or later, and/or nhc98-1.16/hmake-3.06 or later, and/or
Hugs98 (Sept 2003) or later. You must also first download and
install the polyparse
package as a pre-requisite.

Then, for more recent compilers,
use the standard Cabal method of installation:

to configure, build, and install HaXml as a package for your
compiler(s). You need write permission on the library installation
directories of your compiler(s). Afterwards, to gain access to
the HaXml libraries, you only need to add the option -package
HaXml to your compiler commandline (no option required for Hugs).
Various stand-alone tools are also built - DtdToHaskell, Xtract,
Validate, MkOneOf - and copied to the final installation location
specified by the --prefix=... option to configure.

To build/install on a Windows system without the Cygwin shell and
utilities, you can avoid the configure/make steps by simply using the
minimal Build.bat script. Edit it first for the location
of your compiler etc.

Version 1.20.2 adds --version and --help arguments to all executables.
Version 1.20.1 fixes a small bug in DtdToHaskell.
Version 1.20 is the new stable version.

Version 1.19.7 requires package polyparse-1.2 (previous versions require
polyparse-1.1).
Version 1.19.6 has some bugfixes to XmlContent, to ensure that types
created via DtdToHaskell get converted back to XML correctly.
Version 1.19.5 has some bugfixes to DtdToHaskell, to cope with especially
large datatypes.
Version 1.19.4 has some minor bugfixes to increase compatibility with the
ByteString package.
Version 1.19.2 updates the Cabal file to enable HaXml to build with both
ghc-6.6.x and ghc-6.8.x.
Version 1.19.1 fixes a build error in 1.19.
Version 1.19 improved the lazy XML parsing, and fixed some space leaks
in the XtractLazy tool.

Version 1.18 pulled out the parser combinator libraries as a separate
package (called polyparse), which must now be downloaded and installed
before installing HaXml.

Version 1.17 essentially just fixes compatibility with ghc-6.6.
However, it also include a lazier pretty-printer to use in conjunction
with the lazy parser, to save running out of memory on large datasets.

Version 1.16 adds laziness to the parser combinator libraries, such that
they can start to return partial results before a whole entity has been
parsed. Partial is also used in the sense that the returned value can
contain bottom - an error which gets thrown as an exception when you try
to explore the inner regions of the value. In terms of XML, it means you
get an element back as soon as its start-tag has been consumed, but if
there are parse errors later on, BOOM. However, if there are no errors,
it does mean that your processing will be (a) faster and (b) less memory
hungry. Another cool thing is that, even in the presence of errors, you
still might get enough output to satisfy your processing task before the
error is noticed.

Use Text.XML.HaXml.ParseLazy and
Text.XML.HaXml.Html.ParseLazy to try it out. There are also
lazy versions of the supplied demo programs: CanonicaliseLazy
and XtractLazy.

Version 1.15 is essentially 1.14 with some bugfixes, and some new
functionality, especially in the parser combinator libraries. DrIFT now
supports deriving the XmlContent class, and DtdToHaskell now also
derives the XmlContent class, in addition to determining a collection of
Haskell datatypes equivalent to a given DTD.

Error messages from parsing are much improved in 1.15 - they should
locate any error far more specifically and accurately. Let me know
about examples which do not report correctly.

Prior to 1.14, there were two separate classes, Xml2Haskell and
Haskell2Xml. They are now combined into the single class XmlContent.
Make sure you get a recent version of DrIFT if you want to derive this
class from Haskell datatypes - the included version of DtdToHaskell has
not yet been updated for deriving the class the other way, from an XML DTD.

Version 1.14 also contains a new SAX-like stream parser.

A while back, Graham Klyne extended the 1.12 version of HaXml
significantly, in particular to ensure that the parser passes a large
XML acceptance test suite, and to deal more correctly with Unicode,
namespaces, and parameter entity expansion. His modifications will
eventually be merged back in to the main CVS tree, but in the meantime,
you can get his version here:
http://www.ninebynine.org/Software/HaskellUtils/

The previous stable version (1.13) had the following features and fixes:

Development of these XML libraries was originally funded by Canon
Research Europe Ltd.. Subsequent maintenance and development has
been partially supported by the EPSRC, and the University of York.

Licence: The library is Free and Open Source Software,
i.e., the bits we wrote are copyright to us, but freely licensed
for your use, modification, and re-distribution, provided you don't
restrict anyone else's use of it. The HaXml library is distributed
under the GNU Lesser General Public Licence (LGPL) - see file
LICENCE-LGPL for more details. We allow one
special exception to the LGPL - see COPYRIGHT.
The HaXml tools are distributed under the GNU General Public Licence
(GPL) - see LICENCE-GPL. (If you don't
like any of these licensing conditions, please contact us to discuss
your requirements.)

Joe English has written a more space-efficient parser for XML
in Haskell, called hxml. What is more, it can be used as a simple
drop-in replacement for the HaXml parser!
Available here.

Uwe Schmidt designed another
Haskell XML Toolbox
based on the ideas of HaXml and hxml. It is well-maintained, and has
recently been updated to use arrow-based combinators rather than filters
as in HaXml.

To use HaXml and HXT together, Henning Thielemann has put together
WraXML,
a wrapper using an alternative tree data structure, together with
conversions to/from HaXml and HXT.

Andreas Neumann of the University of Trier has written a
validating XML parser in Standard ML:
here.

Erik Meijer and Mark Shields have a design for a functional programming
language that treats XML documents as basic data types:
XMLambda.

Benjamin Pierce and Haruo Hosoya have a different but similar design in
XDuce, which is
also implemented.

Taking XDuce's approach further, is the very cool
CDuce by Véronique Benzaken,
Guiseppe Castagna, and Alain Frisch. The CDuce language does
fully statically-typed transformation of XML documents, thus
guaranteeing correctness, and what is more, it is also faster
than the untyped XSLT!

The Xcerpt project uses HaXml
to create another rule-based query and transformation language for XML,
inspired by logic programming, and based on positional selection rather
than navigational selection.

The Java world has adopted the ideas from DtdToHaskell into
the Java Architecture for XML Binding
(JAXB). JAXB translates
an XML Schema Definition into a set of Java classes, and provides
the runtime machinery (like XmlContent) for reading and
writing objects of those classes to/from XML files.

There is a comprehensive reading list for XML and web programming in
functional languages here.